Printed Circuit (PC) boards, also called printed wiring boards, are known in the art. Integrated Circuit (IC) chips are mounted on PC boards and interconnected through PC board wiring. These IC chips may be packaged and, the packaged chips mounted on a board or, to achieve higher chip density, chips may be mounted directly onto the board.
To further increase the number of chips that may be mounted on PC boards, such as state of the art Surface Laminar Circuit (SLC) boards, narrower wires are crowded closer together on each wiring layer to wedge more and more wires into smaller and smaller areas. Currently, the space between wires is about 5 mils (12.5 .mu.m). One result of increasing wiring density is a corresponding increase in PC board costs.
So, to control PC board costs and to reduce waste, defective PC boards are repaired, whenever possible. Excimer lasers, which are expensive and difficult to maintain, are normally used to repair surface wiring on ceramic substrates.
However, a typical excimer laser used to repair a short on a ceramic surface would pass through an organic surface layer of an SLC board, damaging underlying wiring. Also, Xenon lasers with a wavelength in the blue-green range (450-570 nm) have been found to pass through an organic surface layer, damaging any underlying wires.
Thus, these types of lasers may not be used to repair defects (especially shorts) in dense SLC surface wiring. Instead, shorts are repaired on SLCs, typically, by cutting them with a knife (manually) or with an ultrasonic cutter.
Unfortunately, cutting the short may also damage the PC board being repaired. The knife may slip and damage the SLC. Further, as wiring density increases, cutting becomes very difficult. So, for dense wiring on an organic insulator, such as an SLC, manual repair is inconsistent at best.
Thus, there is a need for a method of removing shorts on densely wired PC boards, such as SLCs.